Pros

Pro

Robust integration with a huge number of tools

Slack integrates with tools like Trello, GitHub, Dropbox, Mailchimp, and dozens of others, so you can have a centralized event feed of your project right alongside your chat. This is tremendously useful for keeping context with your discussions.

Pro

Very polished user experience

The entire Slack interface is polished and intuitive to use. There are very few bugs or inconsistencies in the UI and it's very fast to use. There is nothing in particular that is new with Slacks implementation of team chat, but the execution of the groups (called channels), search, external service integration and notifications is close to perfect.

Pro

Native apps for iOS, Android, Windows Phone, OS X, Linux, and Windows

Slack has fully native apps for iOS, Android, Windows Phone, OS X, Linux, and Windows to give you the full functionality of Slack on most major platforms.

Pro

Drag & drop files in channels

You can upload a file to any channel over HTTPS simply by dragging and dropping.

Pro

@mentions

You can ping people to get their attention even if they are not online by @mentioning them. Slack supports desktop notifications.

Pro

Fantastic search functionality

You can deep search messages, files and snippets. Given Slacks integration into many external services, Slack is good enough to act as the central search interface for your entire team.

Pro

Flexible, granular notification settings

Notifications are handled separately for mobile and the web app. You can receive notifications for all messages, just direct messages, or based on filters, and you can have different settings for different channels: you don't have to get notified every time someone pushed to GitHub or every time someone posts to off-topic chat, unless you want to.

Pro

Syntax coloring

Pasted code can be colored based on syntax.

Pro

Freemium plan

Pro

Inline link previews (photos, mockups, etc)

Pro

Slackbot extensible chat robot

The "Slackbot" can is an extensible robot that can be set you remind you about tasks, auto respond to certain phrases and a variety of other functionality.

Pro

Supports multiple teams

You can be signed into multiple teams simultaneously and quickly switch between them.

Pro

Edit messages easily

Pro

Shows local time of each participant

You can click on the profile of a user to see their local time. An especially useful feature when members of your team are working in different timezones.

Pro

Emoji reactions to limit excessive posts and notifications

Pro

It can surprise you

There's a checkbox in preferences under advanced options that may surprise you.

Pro

Dev team is invested, responsive, and friendly.

Having submitted both feedback and support requests for bot development, I can personally attest that the team takes feedback seriously, and responds quickly to communication. This is vital for any closed-source or hosted project.

Pro

IRC connectivity over SSL

Cons

Con

Expensive when you need to upgrade

At $6.67 per user / month (or $8 if billed monthly) , Slack is significantly more expensive than the competition if you need features such as unlimited integrations (more than 10) or unlimited message storage (more than 10,000). However, the free version of Slack includes unlimited users.

However if you need only unlimited messages you can use storage services like https://slarck.com to upload then browse and search your entire message history, while staying in Slack's free plan. So with a combo of Slack+Slarck this con is not that major.

Con

No self-hosting available

If you ware worried about third-parties getting access to your data you should consider self-hosting. With self-hosting you are in control over where your data is stored, who has access to it. You will also not be vulnerable to exploits of a third-party provider.

Con

No threaded conversations

Other tools like Co-Meeting include threaded conversations, that can be useless when more structure is needed, for instance when everyone posts their ideas and people want to comment on individual ideas.

Con

Hidden max limit of free users per channel

Slack says that their free accounts support an unlimited number of users, which is true. However they don't mention that there is an undisclosed maximum number of users per channel (8462). For a large open source community, this is something to keep in mind.

Con

"Native" desktop apps are web apps

While it's great that Slack provides installable apps for Windows, OS X, and Linux, they're just the Slack web app wrapped in Electron shell. This means they don't offer the same level of native UX that a truly native toolkit app would.

Con

API doesn't allow custom widgets in chat.

Con

No built-in video chat yet

While Slack has no built-in video chat, it integrates with Hangouts and enable such functionality. Additionally, recent acquisition of Screenhero promises eventual built-in support for audio and video conferencing.